Many endogenous peptides of mammalian and amphibian origin bind to specific opioid receptors and elicit an analgesic response similar to classic narcotic opiates. Many different types of opioid receptors have been shown to coexist in higher animals. For example, see W. Martin et al., J. Pharmacol. Exp. Ther., 197, p. 517(1975); and J. Lord et al., Nature(London), 257, p. 495(1977). Three different types of opioid receptors have been identified. The first, .mu., shows a differentiating affinity for enkephalin-like peptides. The second, .delta., shows enhanced selectivity for morphine and other poly-cyclic alkaloids. The third, .kappa., exhibits equal affinity for either group of the above ligands and preferential affinity for dynorphin. In general, the .mu.-receptors seem to be more involved with analgesic effects. The .delta.-receptors appear to deal with behavioral effects, although the .delta. and the .kappa.-receptors may also mediate analgesia.
Each opioid receptor, when coupled with an opiate, causes a specific biological response unique to that type of receptor. When an opiate activates more than one receptor, the biological response for each receptor is affected, thereby producing side effects. The less specific and selective an opiate may be, the greater the chance of causing increased side effects by the administration of the opiate.
In the prior art, opiates, opioid peptides, and analogs thereof, have either failed to demonstrate, or have demonstrated a limited degree of specificity and selectivity for the type of receptor, or receptors, to which they bind.
The primary site of action of analgesic opioids is the central nervous system (CNS). Conventional narcotic analgesics are normally quite hydrophobic and thus are extremely well-suited to permeate lipid membranes, such as the blood-brain barrier. Due to this physical capability, analgesics tend to bind with opioid receptors within the central nervous system in the brain. However, they do not necessarily bind with a homogeneous receptor subtype. This binding causes medically undesirable side effects to occur.
Opiates can cause serious and potentially fatal side effects. Side effects such as respiratory depression, tolerance, physical dependence capacity, and precipitated withdrawal syndrome are caused by nonspecific interactions with central nervous system receptors. See K. Budd, In International Encyclopedia of Pharmacology and Therapeutics; N. E. Williams and H. Wilkinson, Eds., Pergammon: (Oxford), 112, p.51 (1983). Therefore, opioid analgesics acting principally through opioid receptors in the peripheral nervous system would not be expected to cause similar unwanted side effects as those side effects associated with opioid analgesics affecting the central nervous system.
To date, one of the few classes of agents known to exert peripheral analgesic effects are non-steroidal anti-inflammatory agents, such as aspirin, ibuprofen, and ketorolac. These agents do not interact with opioid receptors but are known to inhibit cyclooxygenase and attenuate prostaglandin synthesis. These weak analgesics do not have centrally mediated side effects, but they can cause other side effects such as ulcerations of the gastro-intestinal tract.
There is therefore a need for opioid analgesic that can substantially affect the peripheral nervous system and therefore overcome some of the disadvantages of conventional opiates by substantially preventing unwanted side effects from occurring.
There is therefore a need to provide opioid-like peptides which act peripherally but substantially avoid the unwanted side effects associated with conventional peripherally acting analgesics.
There is also a need for opioid analgesic that can be administered orally.